4 Answers
4

For starters, the vast majority of apps, of the million out there, do not sell well, whether open or closed source. So don't expect any app to sell well unless it is a stand-out app and well marketed.

For iOS apps, someone would need to have paid Apple $99/annum for an iOS Developer program enrollment, plus have a Mac, in order to be able to download your open source, build the app, and load it on their device, without buying your app. Not many end users would do this, even if they had the expertise.

However, there is little or nothing to prevent some other developers from doing this, even submitting it to an app store under their own name, perhaps undercutting your price, or adding advertisements to a free app.

There are more and more developers doing the latter. Some reportedly even using content that is copyrighted and not open source. So competition is almost guaranteed if your app gains any visibility. Your advantage over the copycats, if any, will be brand name recognition from marketing, PR, or fame, if you have any, as many of the app stores do attempt to protect registered trademarks.

ADDED:

If you distribute your open source under the GPL v3 license, there may be some legal opinion that using code so licensed is incompatible with Apple's iOS App store DRM. But you as the 100% copyright holder could submit your own app to the iOS App store yourself, since the license doesn't apply to the owner. However, you may be able to consult an attorney about whether you would have grounds to sue anyone else who took your code under GPL v3 license, and tried to do likewise. This might cost you money to enforce though. But IANAL, so consult a lawyer if you want to think about using this strategy.

Your mobile application being open source will not have much of an impact on the sales of the app if it is on the market.

There is a lot of work involved in doing a checkout (you need the relevant SCM installed) and getting the appropriate SDK set up (the relevant license acquired if it is iOS or Windows). That is provided that your target users are that knowledgeable to know that this is what they need to do to get it for free.

It is actually a positive upto a certain level. Providing source may actually be a bonus and would tilt many a advanced user to your side. These users can extend or build apps alongside your application that uses your application features like Activities in android.

Making the source available is not by itself going to attract contributors. The app needs to attract sufficient attraction before that happens.

If you're concerned about competition, then it's best to not open source at all. It is very hard to establish that your featuers were copied and time consuming to get that issue settled.

Often we don't pay/donate because we are lazy and even want to but postpone indefinitely

An example if you don't mind. It's not about mobile, but the concept is interesting. It's ssh server for Windows, called KpyM. The app is open-sourced and the download version contained 5-seconds delay for any client connection if not registered. This practice is usual for closed sharewares, but this was the first time I met it in OSS. And finally I paid. Because

I could live with this reminder, but it reminded me that the job is done and it's better for it to be rewarded. The more I feel the program worked as expected, the more this delay was about wish to pay (because if it's irritation, I can always modify the sources and recompile)

I could recompile without the delay, but I had strong urge not to do. Not mentioning recompiling it and upload this version somewhere for ethical reasons. Hacking closed binaries is illegal, but sometimes has reasoning (for example, solving a complex task), but changing the source code of an OSS program in similar cases won't even get you a badge of a smart guy :)